The railroad-highway crossing, at a common grade, presents a potentially dangerous situation. Highway crossing warning systems have heretofore been developed to provide a warning to highway users of the approach of a train, with the desired goal of insuring that the crossing is clear at the time the railroad vehicle passes thereover. The problem of providing a safe and effective warning system is complicated by a number of variable factors.
AAR recommendations suggest that a minimum 20 second warning time be given of the approach of a train. Because the highway crossing warning system has no control over the speed of the approaching railroad vehicle, it must accommodate its operation to the motion of the railroad vehicle which can slow down or speed up as it approaches the crossing, indeed, the vehicle can even stop and start up again, such motion can be toward or away from the crossing. Furthermore, after the railroad vehicle has passed the crossing, the railroad vehicle may slow down, speed up, stop and then even reverse its motion and re-cross the crossing. The ideal highway crossing system should provide a minimum warning time regardless of these variations.
Further complicating the design of these systems is the variability which is inherently present under normal operating conditions. For example, one typical method of detecting the presence of a vehicle is the track circuit. The track circuit employs a source of electrical energy which is applied to the track rails at one point and an electrical energy detector, such as a relay or other receiver, which responds to the energy impressed on the rails by the transmitter. The presence of a conventional railroad vehicle, with the steel wheel shunt it provides, alters the energy detected at the receiver, and this alteration is usually employed to signal the presence of a train. The track circuit is, however, subjected to variables other than the presence or absence of the train. For example, the track circuit is shunted through the ballast on which the rails are supported. This effective shunt is variable depending, for example, on moisture conditions. Furthermore, the conductivity of the track rails themselves may change their conductivity characteristics due to a variety of factors. One such factor, for example, is the presence or absence of rust in local spots on the rail.
Another type of arrangement which has recently become popular in highway crossing warning systems is the train motion detector. Whereas the track circuit employed the gross change in track circuit conditions caused by a train entering or leaving the track circuit to detect the presence or absence of the train; the motion detector, instead, relies upon the voltage variations at a receiver, as a train approaches or leaves the point at which the receiver is connected to the track rails, to detect train approach or departure. That is, train velocity is implied from the rate of change of voltage detected by the receiver. The variable factors affecting the track circuit also affect this type of operation.
Many of the older highway crossing systems employed insulated track sections. With the popularity of the welded rail, and the associated desire of the railroads to eliminate insulated joints, however, there is a desire to use non-insulated track circuits in the highway crossing warning system. As those skilled in the art will appreciate, the lack of insulated joints provides further variable factors inasmuch as now the changes in weather conditions can not only affect the nominal operating points, but can also affect the "range" within which vehicles can be detected.
From the foregoing discussion, it should be apparent that the design of a highway crossing signalling system is not as simple as it might first appear. Although there are differing views in the field, one school of thought requires that a motion detector type of highway crossing system only be employed with what is termed "wrap-around" protection. The wrap-around protection comprises at least a pair of track circuits (which may be uninsulated), on either side of the highway crossing. Initially, an approaching train is detected by one of these two circuits which may first energize the crossing signal. The motion detector is then allowed to extinguish the highway crossing warning based on train approach speed if, and only if, train motion is first detected. That is, if the train speed and distance are such as to provide more than the necessary minimum warning time, the operation of the motion detector will allow the crossing signal to be inhibited. Of course, as the train approaches the highway crossing, there will come a point in time when only minimum warning time remains; at this point, the motion detector again energizes the highway crossing warning. Thus, the motion detector is not allowed to be effective unless and until it proves its operability by detecting approaching motion of a vehicle whose presence has already been detected by other apparatus. This arrangement has proved to be useful and effective.
However, since the train motion is completely uncontrolled by the warning system, and is only limited by the physical limits on acceleration and deceleration imposed by available torque and braking forces, the conventional arrangement briefly outlined above still cannot operate to assure minimum warning time. Consider, for example, the case of a train which approaches the highway crossing at a speed high enough to be detected and then slows to a relatively slow speed. The wrap-around circuits will sense the presence of the train and initiate a warning. The motion detector, after sensing the higher velocity then "sees" the low velocity and relatively great distance between the train and the highway crossing, determines that, at the present train speed, more than ample warning time remains and so it disables the warning. Train approach continues at a relatively slow speed until, just prior to the time the train is close enough to the highway crossing to provide for minimum warning time at the present velocity of the train, it suddenly begins accelerating. Since the highway crossing warning system can only respond to the perceived motion of the train, and it cannot predict changes in train velocity or acceleration, obviously less than minimum warning time will be provided.
In order to overcome problems of this sort, prior art highway crossing warning systems have been arranged with a timer which cancels the warning a predetermined time after it began if the train does not cross in that period, see, for example, Tsiang U.S. Pat. No. 2,850,623 and Hillig U.S. Pat. No. 2,863,993. Since, however, the predetermined time is unrelated to train parameters, it is apparent that it would be a rather simple matter to "fool" the warning system and prevent it from giving the minimum desired warning time.
It is also common in highway crossing warning systems to provide an "island" track circuit which includes the track rail portions actually crossing the highway. The "island" circuit has a separate train detector which is arranged to actuate the warning if it is not already actuated when the "island" becomes occupied. At first it might seem the problems outlined above could be obviated by merely extending the "island" far enough to provide minimum warning for the maximum velocity trains are allowed to achieve. While such solution is effective in giving the minimum warning, it is not an effective solution for then slower trains cause a warning which is unnecessarily long. This is considered inappropriate for it "teaches" the highway user to ignore the warning and is thus an inappropriate solution.
A further problem which must also be addressed in such systems is the motion detector threshold. Preferably the threshold is proportional to distance, that is a higher speed train will be detected further from the crossing than a lower speed train. While the threshold can be fairly easily fixed to some determined slope, the threshold characteristic will vary and this variability must not allow minimum warning time requirements to be violated.
It is therefore one object of the present invention to provide, in a highway crossing warning system, including a motion detector and wrap-around protection, a ring sustain timer whose time delay is arranged to provide minimum warning time under almost any combination of train approach velocities, and practical accelerations or decelerations. It is another object of the present invention to provide a method and apparatus of providing a highway crossing warning with clearly defined parameters so that minimum warning time is provided under almost all conditions.